Instruments for measuring and displaying speed or velocity of a marine vessel through water have utilized a wide variety of sensors (See, for example, U.S. Pat. No. 4,206,637--paddlewheel interrupts photodetector; U.S. Pat. Nos. 3,531,948; 3,496,770; 3,457,782--paddlewheel with magnet on each paddle or paddle made of magnetic field permeable material which is sensed by electromagnetic pick-up coil; and U.S. Pat. No. 3,706,224--paddlewheel with magnet on each paddle which interrupts current flow in a circuit).
Similarly, numerous instruments exist for providing sonic signals for depth sounding or fish locating. (See, for example, U.S. Pat. No. 4,110,727 and various references cited therein.)
It has also been found helpful for marine navigation and fish finding to measure water temperature by various means.
A need exists, however, for a low cost, instrument which will provide information with respect to all three parameters of speed, temperature and distance in a convenient integrated assembly for ease in mounting.
For the most part, prior art speedometers are mounted through the hull of the vessel requiring the drilling of a large hole in the hull and the attendant sealing problems (See the knotmeter of U.S. Pat. No. 3,531,988). Additionally, with the exception of the transom mounted speedometer, it is customary to completely enclose the paddlewheel speed sensor except for a small portion in which the paddle extends into the water (See U.S. Pat. No. 3,457,782, for example). In such "closed cavity" devices, the speed sensor becomes non-linear at higher speeds of, for example, in excess of 35 miles per hour.
Accordingly, a need exists for an integrated sensor system which will enable display of speed, water depth and water temperature over a wide range of parameters and especially at speeds in excess of about 35 miles per hour.
Lastly, it is important that the sensed signals of speed, temperature and depth should be adequately isolated from each other to prevent interference between signals and resultant display error. This is especially relevant in the case of interference between the relatively small echo return signals received from sonic transducer signalling. In an integrated assembly which includes all three sensors, it is difficult to suppress such interference.